Idaho Republican Rep. Mike Simpson fired back this week at his primary rival, attorney Bryan Smith, escalating one of the most high-profile House primaries of the 2014 elections so far.

Last week, Smith went on the radio airwaves with an ad attacking Simpson — a close ally of Speaker John A. Boehner — for being "liberal," a charge Simpson's campaign denied. On Tuesday, Simpson sent out his own negative radio spot. The minute-long advertisement includes audio of a car crash and an ambulance, before the narrator accuses Smith of being a "personal injury lawyer."

Simpson campaign manager Brody Aston wrote in an email that the ad was "a significant district wide buy" but declined to give a value. But two GOP sources who monitor Idaho radio ad buys told CQ Roll Call the radio buy is worth $6,000 to $6,500.

Simpson marks the Club for Growth's first primary target of the cycle. The wealthy conservative group gave Smith their backing in its first challenger endorsement of the cycle. However, Smith told CQ Roll Call in a recent video interview that he decided to run independently of the club.

"Idaho Republicans will not be fooled by Mike Simpson's desperate radio ads," Smith campaign manager Carrie Brown responded in an e-mail inquiry to CQ Roll Call about the spot.

Also on Tuesday, Smith picked up an endorsement from the Madison Project, a political action committee run by former Rep. Jim Ryun, R-Kan.

Idaho's 2nd District is rated a Safe Republican contest by Rothenberg Political Report/Roll Call.